1. Field of Invention
The present invention is directed to pulse oximetry. More particularly, the present invention is directed to a method and apparatus for determining oxygen saturation in arterial blood by comparing the absorption of two wavelengths of light.
2. Description of Related Art
Presently, pulse oximetry is used for measuring oxygen saturation in arterial blood by comparing the absorption of signals such as two signals of different wavelengths of light. For example, the absorption of infrared and red light is compared to determine oxygen saturation. Red and infrared light are often used because the relative absorption of these wavelengths is dependent on the relative concentration of oxygenated hemoglobin in blood. According to one arrangement, red and infrared light emitting diodes (LEDs) are placed on one side of a finger and a photo-detector is placed on the opposite side. The red and infrared LEDs are flashed in quick succession and the photo-detector records the amount of light received from each LED in turn. During this process, every normal beat of the heart delivers a pulse of blood throughout the body. The pulse of arterial blood in the capillaries between the LEDs and the photo-detector causes the received levels to vary according to the absorption of the light by the changed path length of the blood through which the light passes. The relative percentage change in the two light levels, expressed as a ratio of the changes of level divided by the ratio of levels, is a characteristic of a particular oxygen saturation, if there are no other sources of variation in absorption.
Thus, pulse oximetry has become a widely used clinical variable. Unfortunately, as the usage has spread to different situations, the accuracy of the underlying assumption that the changes in absorption are due only to pulses of arterial blood has become less reliable. For example, it is now common to monitor oxygen saturation by means of pulse oximetry in both ambulatory patients and infants. In both instances, motion of the patient often creates a motion artifact by introducing changes of the path length of light from the LEDs, which causes changes in the detected light levels. In the absence of a means to distinguish between the changes caused by this motion artifact and the true signal from the pulses of arterial blood, the inferred oxygen saturation becomes unreliable.
Presently, there are methods that attempt to obtain more accurate oxygen saturation measurements than the use of the above process alone. One method is the use of bandpass filtering with conventional pulse oximetry. This method is technologically simple and relatively inexpensive. The use of bandpass filters can be used to reduce some types of artifact. For example, shivering produces artifacts predominantly higher in frequency than the pulse frequency. Thus, low pass filters may help to avoid incorrect results due to shivering patients. Similarly, artifacts of a predominant frequency lower than 0.5 Hz may be reduced by high pass filtering. Unfortunately, the absence of methods to reject more general artifacts causes this type of oximetry method to be either unreliable or unusable in many applications.
Another method for obtaining more accurate oxygen saturation measurements is the use of statistical weighting to reduce the influence of artifacts. In this technique, a measure of noise is used to assign weights to a cluster of saturation measurements with the greatest weight being assigned to the measurements with the smallest noise. This cluster may be derived for either sequential blocks of time, or from multiple spectral components. In cases of intermittent artifacts, statistical weighting can be effective. Unfortunately, if the artifacts are continuous, with no data segment providing a result unaffected by the artifacts, the cumulative result is no better than conventional pulse oximetry.
Another method for obtaining more accurate oxygen saturation measurements is the use of adaptive noise cancellation on the detected signals. Such a method is disclosed in Diab et al., U.S. Pat. No. 5,482,035, issued Jan. 9, 1996, and the corresponding family of patents. Such a method involves generating a reference signal from two detected signals. The reference signal is then used to remove undesired signal portions via an adaptive noise canceler such as a joint process estimator. Unfortunately, this method is expensive and requires a relative large amount of power to operate the required signal processor. Also, this is an indirect method for determining the oxygen saturation. For example, this method must perform additional processing on the signal after canceling the noise to obtain the oxygen saturation.
The present invention provides a method and apparatus for determining oxygen saturation in arterial blood. The apparatus includes a detector that is configured to sense a first signal of a first wavelength, the first signal having a first arterial blood component and a first noise component. The detector is also configured to sense a second signal of a second wavelength, the second signal having a second arterial blood component and a second noise component. The first arterial blood component is related to the second arterial blood component by an arterial blood absorption ratio and the first noise component is related to the second noise component by a noise absorption ratio. The apparatus also includes a controller that is configured to determine a value of the noise absorption ratio that maximizes the magnitude of an autocorrelation function. The controller is further configured to determine the oxygenation of blood from the noise absorption ratio.
The controller can further be configured to determine the oxygenation of blood by determining the arterial blood absorption ratio from the noise absorption ratio and by determining the oxygenation of blood from the arterial blood absorption ratio. The controller can also be configured to determine the value of the noise absorption ratio by maximizing the magnitude of the autocorrelation function by setting a derivative of the autocorrelation function with respect to the noise absorption ratio to zero.
The controller can additionally be configured to determine the value of the noise absorption ratio by minimizing the mean square of the derivative of the autocorrelation function with respect to the noise absorption ratio and with respect to the arterial blood absorption ratio. The controller can further be configured to determine the value of the noise absorption ratio by determining a normalized autocorrelation function and by determining the value of the noise absorption ratio at a lag where the normalized autocorrelation function reaches a maximum on a specified interval.
The controller can also be configured to determine the normalized autocorrelation function by dividing the autocorrelation function by the autocorrelation function at a time lag of zero to obtain the normalized autocorrelation function. The controller can additionally be configured to determine the value of the noise absorption ratio by determining the best estimate of a pulse-to-pulse interval. The best estimate of a pulse-to-pulse interval can be determined from external information and/or information determined from available pulse oximetry data.
The controller can further be configured to determine the quality of at least one solution of the noise absorption ratio with respect to another solution of the noise absorption ratio based on the highest signal to noise ratio and choose the best solution based on the quality of the solution.
The present invention also provides a pulse oximeter. The pulse oximeter includes a light source configured to provide first signal of a first wavelength and a second signal of a second wavelength. The pulse oximeter also includes a detector configured to sense the first signal of the first wavelength and the second signal of the second wavelength. The first signal has a first arterial blood component and a first noise component and the second signal has a second arterial blood component and a second noise component. The first arterial blood component is related to the second arterial blood component by an arterial blood absorption ratio. Also, the first noise component is related to the second noise component by a noise absorption ratio. The pulse oximeter additionally includes a processor configured to determine a value of the noise absorption ratio that maximizes the magnitude of an autocorrelation function and to determine the oxygenation of blood from the noise absorption ratio.
The processor can further be configured to determine the oxygenation of blood by determining the arterial blood absorption ratio from the noise absorption ratio and by determining the oxygenation of blood from the arterial blood absorption ratio. The processor can also be configured to determine the value of the noise absorption ratio by maximizing the magnitude of the autocorrelation function by setting a derivative of the autocorrelation function with respect to the noise absorption ratio to zero.
The present invention additionally provides a method for determining the oxygenation of blood. The method includes sensing a first signal of a first wavelength, the first signal having a first arterial blood component and a first noise component, sensing a second signal of a second wavelength, the second signal having a second arterial blood component and a second noise component, the first arterial blood component being related to the second arterial blood component by an arterial blood absorption ratio and the first noise component being related to the second noise component by a noise absorption ratio, determining a value of the noise absorption ratio that maximizes the magnitude of an autocorrelation function, and determining the oxygenation of blood from the noise absorption ratio.
The determining the oxygenation of blood step can also include determining the arterial blood absorption ratio from the noise absorption ratio, and determining the oxygenation of blood from the arterial blood absorption ratio. The determining the value of the noise absorption ratio step can additionally include maximizing the magnitude of the autocorrelation function by setting a derivative of the autocorrelation function with respect to the noise absorption ratio to zero.
The determining the value of the noise absorption ratio step can further include determining the value of the noise absorption ratio by minimizing the mean square of the derivative of the autocorrelation function with respect to the noise absorption ratio and with respect to the arterial blood absorption ratio. The determining the value of the noise absorption ratio step can also include determining a normalized autocorrelation function and determining the value of the noise absorption ratio at a lag where the normalized autocorrelation function reaches a maximum on a specified interval.
The determining the normalized autocorrelation function can include dividing the autocorrelation function by the autocorrelation function at a time lag of zero to obtain the normalized autocorrelation function. The determining the value of the noise absorption ratio step can further include determining the best estimate of a pulse-to-pulse interval. The best estimate of a pulse-to-pulse interval can be determined from at least one of external information and information determined from available pulse oximetry data.
The method for determining the oxygenation of blood can additionally include determining the quality of at least one solution of the noise absorption ratio with respect to another solution of the noise absorption ratio based on the highest signal to noise ratio and choosing the best solution based on the quality of the solution.
The present invention additionally provides a method for determining the oxygenation of blood including the steps of transmitting a signal through a medium containing blood, sensing a first signal of a first wavelength, the first signal having a first arterial blood component and a first noise component, sensing a second signal of a second wavelength, the second signal having a second arterial blood component and a second noise component, the first arterial blood component being related to the second arterial blood component by an arterial blood absorption ratio and the first noise component being related to the second noise component by a noise absorption ratio, determining a value of the noise absorption ratio that maximizes the magnitude of an autocorrelation function, and determining the oxygenation of blood from the noise absorption ratio.
The method can also include the steps of determining the quality of at least one solution of the noise absorption ratio with respect to another solution of the noise absorption ratio based on the highest signal to noise ratio and choosing the best solution based on the quality of the solution. The at least one solution can include a method including the step of minimizing a mean square of the autocorrelation function and the another solution can include a method including the step of normalizing the autocorrelation function. The at least one solution can also include a method including the step of minimizing a mean square of the autocorrelation function and the another solution can include a method including the step of determining the best estimate of a pulse-to-pulse interval. The at least one solution can further include a method including the step of normalizing the autocorrelation function and the another solution can include a method including the step of determining the best estimate of a pulse-to-pulse interval. The at least one solution can be selected from a set of methods including a method including the step of determining the best estimate of a pulse-to-pulse interval, a method including the step of normalizing the autocorrelation function, and a method including the step of minimizing a mean square of the autocorrelation function.